Systemic Lupus Erytbematosus is an autoimmune disease characterized by the production of antibodies to double stranded DNA (dsDNA). These antibodies are believed to arise because of a breakdown in tolerance. In mice transgenie for the IgM heavy chain of an anti-dsDNA antibody designated R4A, there is a population of anergie dsDNA binding B cells that does not spontaneously secrete antibody, nor can it be activated to do so following B cell receptor (BCR) erosslinking. These B cells can however, be activated to secrete antibody in response to the B cell mitogen, LPS or T cell derived factors. These anergie B cells utilize the Vk1 light chain. In the present proposal we are interested in studying the molecular basis for the negative signaling that occurs in anergie dsDNA binding B cells upon BCR erosslinldng. We are also interested in the aspects of signaling that are altered when tolerance is broken upon LPS activation or activation with T cell derived factors. We have therefore crossed R4A, lgM heavy chain transgenie mice with Vk1 light chain lmockin mice to generate mice with an expanded population of anergic dsDNA binding B cells. We will now examine the activation status of signaling molecules in these anergie B cells upon BCR erosslinking. We will also examine how the signaling profile changes when these anergic B cells become activated following LPS stimulation or stimulation by T cell derived factors. These results will help us understand what kind of signaling can trigger a breakdown of tolerance and lead to autoimmune disease. We have also crossed R4A-Cmu heavy chain transgenic mice with mice overexpressing CD19, a molecule that alters signaling thresholds. A breakdown of tolerance and spontaneous secretion of R4ACmu anti-dsDNA antibody has been observed in these mice. We would now like to examine at the cellular and molecular levels how a molecule that alters signaling thresholds can lead to failed B cell tolerance.